Although experimental results indicate that smaller classes promote higher achievement in early elementary school, the broader literature on class-size effects is inconclusive. This seeming contradiction raises questions about the generalizability of experimental evidence, an issue that this article addresses by examining the effects of class size on achievement in kindergarten with data from a nationwide survey, the Early Childhood Longitudinal Study-Kindergarten Class of 1998-99. To distinguish class-level from individual-level effects, this analysis utilizes hierarchical linear models. In response to concerns about selectivity, teacher fixed-effects models are also estimated. In an effort to understand the inconsistent findings of the past, the authors examine classroom conditions that may affect the link between class size and academic achievement, and also consider whether class size has different effects for different groups of students. The authors find no evidence of class-size effects on student achievement in either reading or mathematics, and results indicate that class size is equally insignificant for students from different race/ethnic, economic, and academic backgrounds. Teacher fixed-effects analyses also yield null findings for class size. Instructional activities offer significant boosts to achievement, but the effects of instruction do not differ between small and large classes. The authors discuss why the small class size advantage evidenced by experimental research might not generalize to nonexperimental, naturally occurring settings throughout the nation. (Contains 6 tables and 15 notes.)